Radar and sonar systems are frequently utilized for measuring the distance between scatterers or reflectors of radiant energy as well as for measuring the radial components of the velocities of the scatterers or reflectors by means of Doppler frequency shifts. These measurements may be obtained with either electromagnetic radiation or sonic radiation. While various wavelengths and modulation patterns are selected for particular situations, in the case of measurements of clear air turbulence, electromagnetic radiation having a wavelength on the order of ten microns, such as is provided by a carbon dioxide laser, is preferred. Such lasers are operated either on a continuous wave (cw) or pulsed basis.
A problem arises when it is desired to obtain good range resolution and good Doppler resolution at close range. As is well known, a precise Doppler measurement requires a cw signal, or at least a long pulse signal. In contrast, good range resolution by a pulsed laser radar requires the use of relatively short pulses. At moderate ranges, a compromise can usually be reached wherein the length of the laser pulse signal is adequately long to achieve good Doppler measurements while still being sufficiently short to permit good range measurements. However, at very short ranges, such as one thousand feet, even a reasonably short laser pulse signal would have a duration longer than the round trip propagation time from the radar to the reflector and back to the radar. The existence of the transmitted and received signal pulses simultaneously at the radar precludes or greatly degrades a range measurement at the short ranges. This problem is particularly acute for measurements in the vicinity of an airport runway since such measurements may well fall within the aforementioned one thousand foot range.